This invention relates to an improved method of estimating seismic velocities for use in generating seismic sections indicating valuable characteristics of subsurface earth formations, and more particularly, to a method in which amplitude versus offset analysis is used to precisely correct velocity estimates made by prior known methods.
Modern seismic prospecting techniques normally employ the common depth point, CDP, or common midpoint, CMP, techniques to improve signal-to-noise ratio. In these methods, seismic signals are generated sequentially at each of a number of points along a seismic prospecting path while reflections are recorded at all of the points following generation of each signal. The recorded signals are then organized into gathers of traces each corresponding to a common depth point or common midpoint. That is, all of the signals in a gather occur from source receiver pairs equally spaced about the point in question along the prospect path. The basic purpose of this exploration technique is to allow the signals within each gather to be combined to improve signal to noise to ratio. However, due to the different path lengths involved in each source receiver pair, corrections must be made to the individual traces within the gather to place them in alignment before stacking. These corrections, known as normal moveout, NMO, corrections depend primarily on estimated velocities of the earth formations through which the signals passed. Errors in the velocity estimation result in errors in the alignment of the signals and thereby reduce the signal to noise ratio of the resulting stacked signal.
The classical normal moveout correction method depends on the maximization of normalized stack power to obtain what appears to be the best fit. These techniques are generally referred to as semblance methods. The use of the semblance methods and further improvements thereto are discussed in U.S. Pat. No. 4,570,246 issued to Herkenhoff, et al., Feb. 11, 1986, which patent is hereby incorporated by reference for all purposes.
In addition to improving signal to noise ratio in conventional amplitude traces, the CDP method of exploration allows estimation of various characteristics of subsurface formations which can be used to predict the hydrocarbon bearing potential thereof. This can be done through the amplitude versus offset, AVO, technique in which the variation in amplitude of signals reflected from given subsurface interfaces is analyzed for changes relating to the angle of incidence or offset between source and receiver pairs. In order to properly perform such analysis, all other sources of amplitude variation should be removed first. For example, the very fact that the signal path length is greater for greater offset pairs naturally reduces signal levels. Path length estimates are in turn effected by estimated seismic velocities. Slight errors in estimated seismic velocities can generate erroneous amplitude changes with offset which are greater than the actual variations resulting from the change in angle of incidence.